Heating resistor type air flow rate measuring apparatus

ABSTRACT

A heating resistor type air flow rate measuring apparatus is provided with a couple of heating resistors placed at the positions where those resistors may each interfere thermally with respect to an air flow, and a couple of driving circuits for driving those heating resistors. The air flow rate signal is obtained by calculating the difference between the output signals of a couple of heating resistors in terms of heat radiation rate effected by an air flow, and adding the difference value onto the output signal of one of the heating resistors.

BACKGROUND OF THE INVENTION

The present invention relates to an air flow meter for measuring theintake air flow rate to an internal combustion engine, especially, aheating resistor type air flow rate measuring apparatus for measuringthe air flow rate in the condition accompanied with a backward flow in apulsating flow.

The air flow in an internal combustion engine is pulsated by acontinuous make and break operation of the intake air valve. Thepulsation is so amplified by the effect of the columnar vibration in theintake air duct, and the air flow inside the intake air pipe becomes abackward flow in specific conditions related to the number of rotationsof the engine and the aperture of the throttle valve. This backward flowbrings about various bad effects in the heating resistor type air flowrate measuring apparatus. As for an apparatus for solving this problem,a prior art apparatus disclosed in Japanese Patent Application Laid-OpenNo. 1-206223 (1989) is known as an air passage structure having a subair duct shaped in a letter I (or L) which is used as a means forincreasing the accuracy of the measurement of the heating resistor typeair flow rate measuring apparatus operated under a condition where abackward flow occurs with a pulsating flow. In this prior art device,the air passage is so configured that the backward flow directly blowsagainst the heating resistor by forming a wall facing against thebackward flow.

As for another apparatus for reducing the bad effect of the backwardflow, a prior art device is disclosed in Japanese Patent ApplicationLaid-Open No. 62-812 (1987). In this prior art device, similarly to thepresent invention, by detecting the direction of the air flow using thethermal interference between a couple of heating resistors, the outputvoltage signals from the heating resistors are altered by judging thedirection of the air flow; when the air flow is a forward flow, theoutput voltage to be used is selected from the output voltage signal ofthe heating resistor for the forward flow; when the air flow is abackward flow, the output voltage signal from the heating resistor forthe backward flow is selected.

In general, it is difficult to measure the direction of the air flow,forward or backward, selectively only by using a single heatingresistor. In order to solve this problem, for example, as shown in FIG.10, in observing the average output of the heating resistor type airflow rate measuring apparatus by varying the boost pressure by makingthe throttle valve gradually open while the number of rotations of theengine is maintained to be constant, the average output of the air flowrate increases linearly as the intake negative pressure increases undera certain threshold value, and for the boost pressure above a certainthreshold value, the average output of the air flow rate is estimated tobe larger than the actual air flow rate (which is designated as anover-shooting phenomena). Though the pulsation in the air flow rate inthe heating resistor type air flow rate measuring apparatus isrelatively small when the throttle valve opens with a small aperture,the amplitude of the pulsation in the air flow rate increases as thethrottle valve gets to open, and finally, at a throttle valve anglelarger than a certain angle (about 30 to 50°) (in the right region of Ain FIG. 10), the pulsation amplitude contains the backward flowcomponent. Thus, when the backward flow occurs, as the heating resistorcan not discriminate the direction of the air flow whether forward orbackward as described above, the average output of the air flow rate isestimated with the forward flow component as well as the backward flowcomponent, and thus takes on larger values.

By means of forming a wall against the direction of the backward flow asdescribed above for the prior art, and making he air passage structureso that the backward flow may not low directly against the heatingresistor, it is possible to reduce the estimation error for the averageoutput. However, the reduced error with this means is only half of theoverall error. This is because the amount of forward flow increases asthe amount of backward flow increases. Thus, in order to reduce theestimation error due to the backward flow, it is necessary to reduce theoutput value of the forward flow when the backward flow occurs orsubtract the backward flow component from the forward flow component aswell as the measurement of the forward flow component. There is a priorart apparatus related to this solution in which, in case the backwardflow is observed by detecting the direction of the air flow by comparingthe output signals from those two heating resistors using a couple ofheating resistors as disclosed in the other prior art described above,the backward flow component is subtracted from the forward flowcomponent. This method has yet another problem. One is related to thereduction of resolution in supplying data to the micro computer. DCvoltage handled by many micro computers used for general automotiveapplications is between 0 and 5.12 (V). However, in this method whereboth the forward flow and the backward flow have a similar relationshipbetween the air flow rate and the output voltage, the resolution of theforward flow is reduced. In an extreme case, the threshold voltage2.56(V) is used and the range below 2.56(V) is for the range of theoutput voltage for the backward flow and the range over 2.56(V) is forthe range of the output voltage for the forward flow. Thus, theresolution of the output voltage for the forward flow in this case ishalf of the resolution when the entire range between 0 and 2.56(V) canbe used for the forward flow. Though the threshold voltage 2.56(V) inthis case is a somewhat extreme case, the resolution for the forwardflow is reduced because the threshold voltage should be determinedbetween 1(V) and 2(V) in order to measure the backward flow precisely.

If the heating resistor has a thermal response delay, the detection ofthe backward flow is delayed when comparing the output signals from theheating resistors. This detection delay has an effect on the measurementprecision. This can be illustrated with FIGS. 11A and 11B; when thebackward flow begins to rise up at point B in FIG. 11A, the outputsignal level of the backward flow does not exceed the output signallevel of the forward flow, and therefore, the existence of the backwardflow is not proved until the output signal level of the backward flowreaches point C; thus, the detection of the backward flow is so delayed.

Further, as disclosed in Japanese Patent Application Laid-Open No.62-812 (1987), the conventional apparatus determines a direction of airflow by using two heating resistors and produces an output signal byusing either one of the detection signals. A noise component produceddue to the mutual interference between the two heating resistors andincluded in the output signal is moderated by attenuating thealternating current component.

However, because the output signal is attenuated in the prior artapparatus, there is the problem that the delay of detection becomeslarge at the time when the air flow is inverted and thus the precisionof the measurement deteriorates.

SUMMARY OF THE INVENTION

Therefore, a first object of the present invention is to increase theprecision of the measurement of the air flow rate in the pulsated flowaccompanying with the backward flow in a practical on-board environment,which is one of the major problems in the above described heatingresistor type air flow rate measuring apparatus, and to provide aheating resistor type air flow rate measuring apparatus which hasadvantages in ease of handling, reliability and cost.

A second object of the present invention is to provide an improvedheating resistor type air flow rate measuring apparatus which can reducethe above-mentioned noise, thus maintaining the precision ofmeasurement.

In order to solve the above-described first problem, a couple of heatingresistors are placed at positions which interfere with the air flow withrespect to thermal properties, and if the air flow is a forward flow,the output signal from the sensor is corrected by the electronic circuitso that the output signal for the forward flow may be equal to theoutput signal for the backward flow, but if the air flow is a backwardflow, the difference between the output signal for the forward flow andthe output signal for the backward flow is so adjusted as to be larger.In addition, the larger one of the output signals from those two heatingresistors is so adjusted as to be equalized to be the lower one, and ifthe air flow is a backward flow, by reducing the output signal of theheating resistor for the forward flow, the overall average value of theoutput signals is thus so adjusted as to be lowered. In this method, thedifference between the output signal for the forward flow and the outputsignal for the backward flow which occurs only when the air flow is abackward flow is used as the correction value. With this method, theswitching operation of the output signals for the forward flow and thebackward flow by using the switching circuit can be eliminated. Andfurthermore, the threshold value for separating the forward flow and thebackward flow is not required and the output voltage used for theheating resistor type air flow rate measuring apparatus can be variedbetween 0 and 5.12(V), and therefore, a higher resolution for the outputsignal can be established when the air flow is a forward flow. As thedifference between the output signal from the heating resistor for theforward flow and the output signal from the heating resistor for thebackward flow necessarily arises when the air flow is a backward floweven if the heating resistors have a thermal response delay, thedetection and judgment of the backward flow can be performed precisely.

Further, the preferable apparatus for attaining the above-describedsecond object is as follows.

A heating resistor type air flow rate measuring apparatus is provided inwhich a forward flow detection signal is detected from a heating currentnecessary to heat to the predetermined temperature a forward flowheating resistor installed in an air passage, and a backward flowdetection signal is detected from a heating current necessary to heat abackward flow heating resistor installed in the air passage to thepredetermined temperature. The apparatus comprises:

a cancelling means for cancelling a differential mode noise included ineach of the detection signals by adding the component of the alternatingcurrent of the backward flow detection signal to the forward flowdetection signal and adding the component of the alternating current ofthe forward flow detection signal to the backward flow detection signal.

Another preferable apparatus related to the second object is as follows.

A heating resistor type air flow rate measuring apparatus is providedwith a pair of air flow rate detecting parts for detecting heatingcurrents necessary to heat a forward and a backward flow heatingresistor installed in an air passage to the predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal, in order to output an air flow rate signal including adirectional component of the air flow in the air passage by using eachdetection signal. The apparatus comprises:

a cancelling means for cancelling differential mode noises included inthe forward and the backward detection signals by adding the componentof the alternating current of the backward flow detection signal to theforward flow detection signal and adding the component of thealternating current of the forward flow detection signal to the backwardflow detection signal, and outputting the forward and the backward flowcancellation signals;

wherein an air flow rate signal is output by using the forward and thebackward flow cancellation signals instead of the forward and thebackward flow detection signals.

A still further preferable apparatus related to the second object is asfollows.

A heating resistor type air flow rate measuring apparatus comprising:

a pair of air flow rate detecting parts for detecting heating currentsnecessary to heat a forward and a backward flow heating resistorinstalled in an air passage to the predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal,

a signal comparing means for determining the direction of the air flowin the air passage by the comparison of large and small of the forwardand the backward flow detection signals,

a signal selecting means for selecting one of the forward and thebackward flow detection signals on the basis of the result ofdetermination, and

a differential amplifying circuit for switching and inputting theforward and the backward flow detection signals, adding an alternatingcurrent component of the backward flow detection signal to the inputforward flow detection signal, and switching and outputting either oneof an output signal higher than a reference voltage in proportion to theadded signal and an output signal lower than the reference voltage inproportion to the input backward flow detection signal;

wherein an air flow rate signal including a directional component of theair flow is output by using the output signal from the differentialamplifying circuit.

A still further preferable apparatus related to the second object is asfollows.

A heating resistor type air flow rate measuring apparatus comprising:

a pair of air flow rate detecting parts for detecting heating currentsnecessary to heat a forward and a backward flow heating resistorinstalled in an air passage to the predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal,

a signal comparing means for determining the direction of the air flowin the air passage by the comparison of large and small of the forwardand the backward flow detection signals,

a signal selecting means for selecting one of the forward and thebackward flow detection signals on the basis of the result ofdetermination, and

a differential amplifying circuit for switching and inputting theforward and the backward flow detection signals, inverting the phase ofan alternating current component of the forward flow detection signaland adding the resultant signal to the input backward flow detectionsignal, and switching and outputting either one of an output signalhigher than a reference voltage in proportion to the forward flowdetection signal and an output signal lower than the reference voltagein proportion to the added signal;

wherein an air flow rate signal including a directional component of theair flow is output by using the output signal from the differentialamplifying circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the heating resistor type air flow ratemeasuring apparatus according to one embodiment related to a firstobject of the present invention.

FIG. 2 shows output signal curves of the individual heating resistors incase of altering the air flow directions when using the thermalinterference between two heating resistors.

FIG. 3 shows output signal curves of the individual heating resistors incase of altering the air flow directions showing one embodiment relatedto the first object of the present invention.

FIG. 4 is a circuit diagram of the output correction part in the heatingresistor type air flow rate measuring apparatus showing one embodimentrelated to the first object of the present invention.

FIG. 5 shows pulsating waveforms of the output signals of the heatingresistors in the existence of the pulsated air flows in the experimentsusing the heating resistor type air flow rate measuring apparatusaccording to one embodiment related to the first object of the presentinvention.

FIG. 6 is a block diagram of the heating resistor type air flow ratemeasuring apparatus according to another embodiment related to the firstobject of the present invention.

FIG. 7 is a block diagram of the system where the signal processingapparatus has a function of the heating resistor type air flow ratemeasuring apparatus showing another embodiment related to the firstobject of the present invention.

FIG. 8 is one structural example of the heating resistor type air flowrate measuring apparatus related to the first object of the presentinvention.

FIG. 9 is another structural example of the heating resistor type airflow rate measuring apparatus related to the first object of the presentinvention.

FIG. 10 shows an over-shooting phenomena of the heating resistors incase of altering the intake negative pressure by opening gradually thethrottle while keeping constant the number of rotations of the engine.

FIGS. 11A and 11B show output signals of the individual heatingresistors at the individual throttle angles in case of using the outputsignal alternation method and in case of using the heating resistorshaving response delay characteristics.

FIG. 12 shows a system controller diagram for controlling the internalcombustion engine by using the heating resistor type air flow ratemeasuring apparatus related to the first object of the presentinvention.

FIG. 13 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a first embodiment related to asecond object of the present invention.

FIG. 14 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to another embodiment related to thesecond object of the present invention.

FIG. 15 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a third embodiment related to thesecond object of the present invention.

FIG. 16 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a fourth embodiment related to thesecond object of the present invention.

FIG. 17 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a fifth embodiment related to thesecond object of the present invention.

FIG. 18 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a sixth embodiment related to thesecond object of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be explainedhereinafter. Firstly, the preferred embodiments related to the apparatuswhich can attain the first object will be described with reference toFIGS. 1 to 12.

FIG. 1 is a block diagram showing the structure of the heating resistortype air flow rate meter as one embodiment of the present invention. Acouple of heating resistors for air flow rate measurement are installedin the intake air duct 18 of the engine. In FIG. 1, the left side of theintake air duct leads to the air cleaner, and the right side leads tothe engine. Therefore, the air flow running in the intake air duct fromthe air cleaner to the engine is defined as the forward flow air flow 4,and the air flow running in the opposite direction is defined as thebackward flow air flow 5.

A couple of heating resistors are placed in the intake air duct, each ofwhich is driven by an independent drive circuit, respectively. Althougha single drive circuit can drive both of the heating resistorstheoretically, there happens to be a thermal response delay with thisconfiguration in which the frequency response for about 20 to 200 Hz cannot be established in a practical engine operation environment, andhence, the direction of the air flow can not be discriminated. Thisdrive circuit is controlled in feedback mode by supplying the heatingflow to the heating resistors so that the temperature difference betweenthe heating resistors and heat-sensitive resistors (the heating andheat-sensitive resistors generally shown as 2 a, 2 b, 3 a, 3 b)installed separately for measuring the intake air temperature may bemaintained at a constant value. Those two heating resistors are placedin positions where the heated air flows interfere thermally with theheating resistors at the upper stream or the down stream of the airflow, respectively. When the forward flow air flow 4 occurs, the heatgenerated by the forward flow heating resistor tries to heat up thebackward flow heating resistor located in the down stream, and when thebackward flow air flow occurs, the heat generated by the backward flowheating resistor tries to heat up the forward flow heating resistorlocated in the upper stream. With this configuration, for example, whenthe forward flow occurs, the heating flow for keeping the constanttemperature difference between the backward flow heating resistor andits corresponding heat-sensitive resistor can be less than the heatingflow for the forward flow heating resistor because the backward flowheating resistor gets the heat generated by the forward flow heatingresistor. Thus, the comparison between the heating flows of those twoheating resistors can teach the direction of the air flow, forward flowor backward flow, and the air flow rate.

FIG. 2 shows the characteristic of the air flow measurement, where theair flow rate is plotted with respect to the output signal from the heatresistors extended in the horizontal axis. Each curve corresponds to thecases for forward flow and backward flow, respectively. As the outputsignal of the individual heating resistors fundamentally corresponds tothe heating flow supplied to the individual heating resistors, when theforward flow occurs, the output signal of the forward flow heatingresistor is larger, and the output signal of the backward flow heatingresistor is smaller. Though the heating flow is so defined as shownabove, the relationship in terms of the output signal voltage can bearbitrarily adjusted by the output control and by the zero span circuitcoupled with the drive circuit.

FIG. 3 shows an example of the output signal characteristic of twoheating resistors used in the heating resistor type air flow ratemeasuring apparatus of the present invention. In the present invention,when the forward flow occurs, the output signal characteristic of theheating resistor for the forward flow and the output signalcharacteristic of the heating resistor for the backward flow areadjusted so as to be approximately equivalent to each other. Thus, theoutput signals from two heating resistors when the forward flow occursare identical to each other, but the difference between the outputsignals shown in FIG. 3 when the backward flow occurs is greater thanthe difference between the output signal shown in FIG. 2. This can beexplained as follows: though the heating flow used in the case shown inFIG. 3 is the same as the heating flow in FIG. 2, the backward flowheating resistor gets the heat generated by the forward flow heatingwhen the forward flow occurs, and the output signal of the backward flowheating resistor is amplified by the zero span circuit so as to makelarger the sensitivity of the backward flow heating resistor to the airflow rate when there is a generic output signal of the backward flowheating resistor. According to this output signal characteristic, theoutput signal can be corrected by the following formula 1 when thebackward flow occurs.

 Vout=Vf−kx(Vr−Vf)+Voffset  (1)

where,

Vout: the output signal of the heating resistor type air flow ratemeasuring apparatus after the backward flow correction,

Vf: the output signal of the forward flow heating resistor,

Vr: the output signal of the backward flow heating resistor,

k: an arbitrary constant value,

Voffset: an offset value for the output signal (defined if necessary),

In the above formula, the term, kx(Vr−Vf) represents the correction termwhen the backward flow occurs. As the output signals of two heatingresistors are the same when the forward flow occurs, the correction termis zero, and the output signal of the forward flow heating resistor isused. On the other hand, as the output signal of the backward flowheating resistor is higher when the backward flow occurs, the componentdue to the backward flow can be corrected. And furthermore, by addingthe constant value k to the difference between the output signals of theforward flow heating resistor and the backward flow heating resistor, aflexible correction can be done. The offset value for the output signal,Voffset, is defined if necessary.

FIG. 4 shows an example of the circuit structure based on the formula 1.This circuit is composed of three operational amplifiers. Thoseoperational amplifiers have their own functions. The output V1 of OP1 isused for supplying the difference between the output signals of theforward flow heating resistor and the backward flow heating resistor(the term (Vr−Vf) in the formula 1). The output V2 of OP2 represents themultiplication of the output V1 of OP1 and the constant value k definedby the ratio between R1 and R2. (kx(Vr−Vf) in the formula 1). The finaloutput Vout of OP3 represents the summation of the output OP2, theoutput signal of the forward flow heating resistor and the offset valuefor the output signal as defined by the formula 1. In FIG. 4, therectangular portion 38 defined with a broken line is an RC filter whichis aimed to eliminate noises in the output signal and to allow theoutput value close to the average of the output signal corresponding tothe amplitude of the pulsating flow to be read into the control unitwithout influence of the sampling timing. The RC filter may be imbeddedinto the signal input part of the engine control unit in the circuits ofthe heating resistor type air flow rate measuring apparatus. Though thiscircuit is basically composed of three operational amplifiers, it ispossible to establish the equivalent circuit composed of two operationalamplifiers by reforming the formula 1. The detailed structure of theequivalent circuit is not shown here.

FIG. 5 is the observation result of the pulsated flow wave forms in thepulsated operation region accompanying a backward flow in the heatingresistor type air flow rate measuring apparatus of the presentinvention. The apparatus is mounted on the actual engine and has thecircuit structure shown in FIGS. 1 and 4 and the output signalcharacteristic shown in FIG. 3. Two curves laying in the lower part ofthe chart represents the output signals of the forward flow heatingresistor and the backward flow heating resistor, respectively, and thesolid line curve represents the output signal of the heating resistortype air flow rate measuring apparatus of the present invention. Theoutput signal of the heating resistor type air flow rate measuringapparatus according to the present invention is defined by theformula 1. For reference, the output signal generated by adding only theoffset value to the output signal of the forward flow heating resistoris shown by the broken line.

Referring to the output signals of the forward flow heating resistor andthe backward flow heating resistor at first, the output signal of theforward flow heating resistor and the output signal of the backward flowheating resistor are almost the same when the forward flow occurs, butthe output signal of the backward flow heating resistor is larger thanthe output signal of the forward flow heating resistor when the backwardflow occurs. Those are output signal characteristics in accordance withthe output signal characteristics shown in FIG. 3. And furthermore,referring to the final output signals, in comparison with the outputsignal generated by adding only the offset value to the output signal ofthe forward flow heating resistor, shown by the broken line, the outputsignal corrected with the backward flow component is almost equal to theoutput signal generated by adding only the offset value to the outputsignal of the forward flow heating resistor when the forward flowoccurs, but smaller when the backward flow occurs. This means that theaveraged output signal when the backward flow occurs can be reduced.Thus, it is proved experimentally that the air flow rate measuringapparatus of the present invention can detect the backward flow, andthat there is such an effect that the output signal of the forward flowheating resistor can be reduced when the backward flow occurs which isthe primary object of the present invention.

FIG. 6 is a block diagram of the heating resistor type air flow ratemeter in another embodiment of the present invention. The basicstructure of the block diagram in FIG. 6 is almost the same as that inFIG. 1. The specific difference from FIG. 1 is that a heater 6 is placedbetween the couple of heating resistors 2 a, 2 b so that the thermalinterference may be established between the heater 6 and the individualheating 2 a, 2 b resistors instead of exchanging heat directly betweenthe two heating resistors. The reason why the structure shown in FIG. 6is used is that, if the distance between two heating resistors is tooshort, the final output signal of the heating resistors is disturbed dueto the heat exchange between the heating resistors even in thesingle-directional air flow. This resultantly leads to noise in theoutput signal of the heating resistor type air flow rate measuringapparatus. It is evident that the heat interference between the heatresistors can not be fully established and hence, the air flow directioncan not be detected if two heat resistors are kept away from each othertoo much.

FIG. 7 is a block diagram of the heating resistor type air flow ratemeter and its output signal processing apparatus in another embodimentof the present invention. The basic structure of the block diagram inFIG. 7 is almost the same as that in FIG. 1. The specific differencefrom FIG. 1 is that the circuit of the heating resistor type air flowrate measuring apparatus only comprises a couple of heating resistorsand the zero span circuit for their output signals, where two individualsignals for the forward flow and the backward flow are supplied to thesignal processing apparatus. Those output signals are processed for thesignal correction and the detection of the air flow direction by thesignal process apparatus. In this embodiment, by enabling a part of thesignal processing apparatus to operate as the signal processing functionof the heating resistor type air flow rate measuring apparatus, there issuch an advantage that the circuit structure of the heating resistortype air flow rate measuring apparatus itself can be simplified.

FIG. 8 is a schematic structural diagram of the heating resistor typeair flow rate measuring apparatus of the present invention. Thestructure contains a circuit board 8 on which the zero span circuit andthe signal processing circuit are integrated, a protector memberincluding a housing member 9 and a cover member 10 for protecting thecircuit board 8, sensor members including the heating resistors 2 a, 2 band the sensing resistors 3 a, 3 b, conductive members 11 forelectrically connecting between the sensor members and the circuitboard, a supporting member for supporting the sensor members and theconnecting members, a sub air route in which the heating resistors areplaced, and a connector part 14 which functions as an interface to theoutside of the apparatus. All of the members are configured as a singlemodule. The sensor part of the module and the sub air passage and othermembers are inserted in the penetration hole 16 of the body member 15. Ahoneycomb lattice 17 is installed in the body member 15 for reducing thedisturbance in the air flow comprising the main air passage in theintake air passage of the combustion engine. The overall structure ofthe heating resistor type air flow rate measuring apparatus is so formedby fixing the module and the body with screws.

FIG. 9 is the structure of the apparatus where the body membercomprising the main air passage of the intake air passage is not formedas part of the heating resistor type air flow rate measuring apparatus,but rather is configured by using the intake air passage composite ductof the combustion engine. In this embodiment, what is used as the bodymember is the composite member of the air cleaner 20 used for removingthe dust in the air supplied into the engine. The body member is placedin the air flow down stream of the air cleaner element 22. A penetrationhole 16 is formed in the air cleaner housing composition member formedwith the duct 23 used as the main air passage for the heating resistortype air flow rate measuring apparatus as a single unit. Thus, thesingle unit containing the heating resistor type air flow rate measuringapparatus and the air cleaner housing composition member 20 is fixedwith screws. According to this structure, by using the existingcomponents, a heating resistor type air flow rate measuring apparatuswithout a newly-built body can be provided at low cost.

Finally, referring to FIG. 12, another embodiment of the presentinvention applied to the internal combustion 25 engine using anelectronic fuel injection system is shown.

The intake air 37 coming through the air cleaner 24 goes through thebody of the heating resistor type air flow rate measuring apparatus 1,the intake duct 25, the throttle body 28 and the intake manifold 29having the injector 30 for injecting the fuel, and reaches the enginecylinder 32. The gas generated in the engine cylinder 33 is exhaustedthrough the exhaust manifold 34.

The air flow rate signal supplied by the circuit module of the heatingresistor type air flow rate measuring apparatus, the throttle valveangle signal supplied by the throttle angle sensor 27, the oxygencontent signal supplied by the oxygen content sensor 35 installed in theexhaust manifold 34 and the engine rotation velocity signal supplied bythe engine tachometer 31 are put into the control unit 36, where anoptimal fuel injection amount and an optimal valve aperture arecalculated and determined by using those signals, and the injector 30and the idle control valve 26 are controlled by using those calculatedoptimal values.

Next, the preferred embodiments related to the apparatus which canattain the second object of the present invention will be described indetail with reference to FIGS. 13 to 18.

FIG. 13 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to one embodiment of the presentinvention. The heating resistor type air flow rate measuring apparatusof the present embodiment is constructed by using air flow ratedetecting parts 48, 58, characteristic adjusting circuits 61, 62, acancelling means 90, a switching circuit 91, and a differentialamplifier 71.

In FIG. 13, the air flow rate detecting parts 48, 58 are provided withWheatstone bridge circuits comprising heating resistors 41, 51, airtemperature detecting resistors 42, 52 and resistors 43, 44, 45, 53, 54,55, and constant temperature control circuits comprising operationalamplifiers 46, 56 and transistors 47, 57 which supply a current to theWheatstone bridge circuits and correct the resistance values of theheating resistors 41, 51 to be constant, in accordance with the value ofthe air temperature detecting resistors 42, 52. The pair of air flowrate detecting parts 48 and 58 detect heating currents for the heatingresistors 41 and 51 by using the resistors 43 and 53 for detecting thecurrents, respectively, and output a forward flow detection signal V2Fand a backward flow detection signal V2R, respectively. The heatingresistors 41 and 51 each have higher detection sensitivity with respectto either one of the forward flow and the backward flow.

In other words, one air flow rate detecting part 48 detects the heatingcurrent necessary to heat the forward flow heating resistor 41positioned in an air passage to the predetermined temperature as theforward flow detection signal V2F, and the other air flow rate detectingpart 58 detects the heating current necessary to heat the backward flowheating resistor 51 installed in an air passage to the predeterminedtemperature as the backward flow detection signal V2R.

The forward flow detection signal V2F and the backward flow detectionsignal V2R have their characteristics adjusted by characteristicadjusting circuits 61 and 62, respectively, and are output as a forwardflow adjustment signal VOF and a backward flow adjustment signal VOR,respectively. After that, these adjustment signals VOF and VOR arecompared with each other by a comparater 69, and the larger signal isselected by a switching circuit 70. Namely, a direction of air flow isdetermined by the comparison of the large and small signals of theforward flow adjustment signal VOF and the backward flow adjustmentsignal VOR in the comparater 69 or signal comparing means.

Finally, a differential amplifier 71 inverses the voltage when thebackward flow is detected, amplifies a forward flow cancellation signalVOUTF or a backward flow cancellation signal VOUTR, which is an outputsignal VOUT, and outputs flow rate signal VG including the component ofthe direction of air flow.

If the heating resistor for detecting the forward flow and the heatingresistor for detecting the backward flow are thermally connected to eachother, then the change in the heating current of each of the heatingresistors has a complimentary relationship. Further, the forward flowadjustment signal VOF and the backward flow adjustment signal VOR havedifferential mode noises of which phases there are inverted with eachother. Similarly to the prior art, if either one of these adjustmentsignals including the differential mode noises is selected and used asan output signal VOUT of the flow meter, then the output signal VOUT andthe flow rate signal VG also include the noises. While the component ofthe differential mode noise can be eliminated by using an attenuationmethod, for example, by averaging the components of alternating currentsof the forward and backward flow adjustment signals, there is theproblem in which the delay of detection becomes large due to theattenuation when the air flow was inverted.

Accordingly, in the present invention, the noise can be eliminatedwithout the delay of detection at the time when the air flow isinverted. Namely, in the embodiment of FIG. 13, the forward flowcancellation output signal VOUTF in which the component of thealternating current of the backward flow signal is added to the forwardflow signal, is obtained from the forward flow adjustment signal VOFadjusted in characteristic by using one cancelling means or circuit 90 acomprising resistors 63, 64 and a capacitor 65. The backward flowcancellation output signal VOUTR in which the component of thealternating current of the forward flow signal is added to the backwardflow signal, is obtained from the backward flow adjustment signal VORadjusted in characteristic by using the other cancelling means 50 acomprising resistors 66, 67 and a capacitor 68. While the cancellingmeans 90 in the present embodiment is constructed by one cancellingmeans 90 a and the other cancelling means 90 b, it does not necessarilyrequire both cancelling means.

A switching means or circuit 91 comprising the signal comparing means orcomparater 69 and the signal selecting means or switching circuit 70,inputs the forward and backward flow adjustment signals VOF and VORadjusted in characteristic which are indicative of a forward and abackward direction of air flow, respectively, and controls the switchingoperation of the switch circuit 70. Namely, the direction of the forwardand backward flow is detected, the switching operation of the switchingcircuit 70 is performed, and the output signal VOUT (VOUTF or VOUTR) ofwhich the noise was cancelled is output. It is possible to use theforward flow detection signal V2F and the backward flow detection signalV2R, instead of the adjustment signals VOF and VOR, respectively.

As described above, it is possible to attenuate or cancel and eliminatethe differential mode noise without a large detection delay at the timewhen the air flow is inverted.

To sum up, in the present invention, the component of the alternatingcurrent of the backward flow detection signal is added to the forwardflow detection signal, and that of the forward flow detection signal isadded to the backward flow detection signal. Namely, the circuit isconstructed so as to cancel the differential mode noises included in twodetection signals by adding the components of the alternating currentsof the respective detection signals opposed to the forward flowdetection signal V2F and the backward flow detection signal V2R to eachother.

In other words, if an air flow meter which can detect a backward flow isconstructed by two air flow rate detecting parts each including aheating resistor, the two heating resistors thermally interfere witheach other as they are thermally close to each other, the excessquantity of heat is transferred to the other heating resistor as aheating current supplied to one heating resistor increases, and theheating current is decreased by the air flow rate detecting part on theside of the heating resistor suffering from the excess quantity of heat.As a result, two heating current detection signals (detection signalsV2F, V2R) include noise components of which the phases are inverted.Accordingly, the present invention is constructed so as to eliminate theabove noise components by adding the components of the alternatingcurrents of the two heating current detection signals to each other.

In the first embodiment, the cancelling means 90 further can include asignal selecting means for selecting either one of the forward flowcancellation output signal and the backward flow cancellation outputsignal.

FIG. 14 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to another embodiment related to thesecond object of the present invention. The second embodiment will beexplained.

In the embodiment of FIG. 14, a differential amplifying circuit 95comprises a circuit having an operational amplifier 76, resistors 63,74, 75, 77 and a reference voltage VCM. The circuit 95 outputs a flowrate signal VG indicative of both the forward flow and the backward flowdirections. The circuit is constructed so as to input the adjustmentsignal VOF or VOR selected by the switching circuit 70 switched by theoutput of the comparater 69, and output a flow rate signal VG indicativeof both directions of the forward and the backward flow and indicativeof the reference voltage VCM when the flow rate is at zero (0). Inaddition, the circuit 95 has a circuit having a resistor 64 and acapacitor 65 for adding in advance only the component of the alternatingcurrent of the backward flow adjustment signal VOR.

Namely, in this embodiment, only the component of the alternatingcurrent of the backward flow adjustment signal VOR is added to theforward flow adjustment signal VOF by an alternating current signalpick-up means having the resistor 64 and the capacitor 65, in order tocancel the differential mode noises. In other words, the cancellingmeans, the switching means and the differential amplifier are formedintegrally in the apparatus according to the present embodiment.

According to the embodiment of FIG. 14, the differential mode noise ofthe forward flow adjustment signal VOF can be effectively decreased.Further, the differential mode noise of the backward flow adjustmentsignal VOR also can be decreased. This occurs when the heating resistor41 for detecting the forward flow is thermally connected to the heatingresistor 51 for detecting the backward flow, because the component ofthe alternating current within the range of the frequency defined by thecircuit having the resistor 64 and the capacitor 65, which acts as anoise filter, is cut off.

Further, if the output voltage of the apparatus shown in FIG. 14 is setto the reference voltage VCM when the flow rate is at zero, it ispossible to operate the apparatus by a single power source for anautomobile.

In this embodiment, the delay of detection of the occurrence of thebackward flow can be improved by correcting the original delay ofresponse in the heating resistor type air flow rate measuring apparatususing equalizers 72, 73 provided at the previous stage of the comparater69 for determining a direction of flow into which the forward andbackward flow adjustment signals adjusted in characteristic are input.It should be appreciated that it is possible to provide the equalizers72, 73 to other embodiments, as well as to remove them.

FIG. 15 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a third embodiment of the presentinvention. The third embodiment will be explained with reference to FIG.15.

In the embodiment of FIG. 15, a differential amplifying circuit 96comprises a circuit having an operational amplifier 76, resistors 74,75, 77, 78 and a reference voltage VCM. The circuit is constructed so asto input the adjustment signal VOF or VOR selected by the switchingcircuit 70 switched by the output of the comparater 69, and output anoutput voltage indicative of the reference voltage VCM when the flowrate is at zero. The circuit 96 also includes a circuit having aresistor 67 and a capacitor 68. In this embodiment, the phase of thecomponent of the alternating current of the forward flow adjustmentsignal VOF is inverted by the circuit having the resistor 67 and thecapacitor 68, and then the component of the alternating current with theinverted phase is input to the operational amplifier 76. The cancellingmeans, the switching means and the differential amplifier are formedintegrally in the apparatus according to the present embodiment

According to the embodiment of FIG. 15, the differential mode noise ofthe forward flow adjustment signal VOF can be decreased, which occurswhen the heating resistors are thermally connected to each other. Thisis because the component of the alternating current within the range ofthe frequency defined by the circuit having the resistor 67 and thecapacitor 68 which acts as a noise filter is cut off. The differentialmode noise of the backward flow adjustment signal VOR also can bedecreased.

FIG. 16 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a fourth embodiment of the presentinvention. The fourth embodiment will be explained with reference toFIG. 16.

In FIG. 16, an air flow meter 80 or the heating resistor type air flowrate measuring apparatus is constructed by the air flow rate detectingparts 48, 58 and the characteristic adjusting circuits 61, 62. Theforward and the backward flow adjustment signals VOF, VOR aretransmitted from the air flow meter 80 to a fuel injection amountcontrol unit 81 for an internal combustion engine. The two adjustmentsignals VOF and VOR in the fourth embodiment are the same as the forwardand the backward flow detection signals in a broad sense.

A signal comparing means 82, alternating current extracting means 83,84, signal adding means 85, 86 and a signal selecting means 87 areprovided inside of the fuel injection amount control unit 81 of a fuelinjection amount control system for an internal combustion engine. Thefuel injection amount control unit 81 produces the output signal VOUTincluding a directional component, and transmit the output signal VOUTto a signal processing means 88 as the information necessary to controlthe fuel injection amount. The signal processing means 88 processes theoutput signal VOUT and makes the flow rate signal VG.

Accordingly, one cancelling means 90 a comprises the alternating currentextracting means 84 and the signal adding means 85, and the othercancelling means 90 b comprises the alternating current. extractingmeans 83 and the signal adding means 86. The switching means 91comprises the signal comparing means 82 and the signal selecting means87. Namely, in this embodiment, the switching means for selecting one ofinput signals or two adjustment signals and the cancelling means foradding the alternating current component of one input signal to theother input signal and adding that of the other input signal to oneinput signal and outputting the resultant signals, are provided in thefuel injection amount control unit 81 which is a semiconductorelectronic circuit.

According to the embodiment of FIG. 16, it is possible to reduce thecost of a fuel injection amount control system, because the signalcomparing means 82, the alternating current extracting means 83, 84, thesignal adding means 85, 86, the signal selecting means 87 and so on canbe integrated into the fuel injection amount control unit 81, and it ispossible to share the arithmetic unit for the fuel injection amountcontrol unit 81 and the above-mentioned means.

FIG. 17 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a fifth embodiment of the presentinvention. This embodiment is different from the embodiment of FIG. 16in that the heating resistor type air flow rate measuring apparatus isnot installed integrally with the fuel injection amount control unit 81.

A cancelling means 90 of FIG. 17 is the same as the circuit having thealternating current extracting means 83, 84 and the signal adding means85, 86 shown in FIG. 16. Further, A switching means of FIG. 17 is thesame as the circuit having the signal comparing means 82 and the signalselecting means 87 shown in FIG. 16.

Further, in FIG. 17, the heating resistor type air flow rate measuringapparatus shown in FIG. 13 is shown in blocks. Namely, the alternatingcurrent extracting means 83 of FIG. 17 corresponds to the alternatingcurrent extracting means having the resistor 67 and the capacitor 68 inFIG. 13. The alternating current extracting means 84 of FIG. 17corresponds to the alternating current extracting means having theresistor 64 and the capacitor 65 in FIG. 13. The signal adding means 85of FIG. 17 corresponds to the signal adding means having the resistor 63and the connection part in FIG. 13. The signal adding means 86 of FIG.17 corresponds to the signal adding means having the resistor 66 and theconnection part in FIG. 13. Still further, the signal comparing means 82of FIG. 17 corresponds to the comparater 69 of FIG. 13, and the signalselecting means 87 the switching circuit 70 of FIG. 13.

FIG. 18 is a circuit diagram showing a heating resistor type air flowrate measuring apparatus according to a sixth embodiment related to thesecond object of the present invention. In the embodiments of FIGS. 13to 18, while the apparatus including the characteristic adjustingcircuits 61, 62, the differential amplifier 71 and so on was explainedas the heating resistor type air flow rate measuring apparatus or theair flow meter 80, it should be appreciated that the heating resistortype air flow rate measuring apparatus means the air flow rate detectingparts 48, 58 may be integrated with the cancelling means 90 in a narrowsense.

Namely, such a heating resistor type air flow rate apparatus has acancelling means for cancelling the differential mode noise componentsincluded in forward and backward flow detection signals by adding thealternating current component of the backward flow detection signal tothe forward flow detection signal and adding the alternating currentcomponent of the forward flow detection signal to the backward flowdetection signal, in addition to means for detecting the forward flowdetection signal V2F from the heating current necessary to heat theheating resistor for the forward flow installed in the air passage tothe predetermined temperature and means for detecting the backward flowdetection signal V2R from the heating current necessary to heat theheating resistor for the backward flow installed in the air passage tothe predetermined temperature.

In this case, the air flow rate signal VG can be obtained by using theforward flow cancellation output signal VOUTF and the backward flowcancellation output signal VOUTR which are output from the cancellingmeans.

In a heating resistor type air flow rate measuring apparatus or air flowmeter comprising an air flow rate detecting part and a characteristicadjusting circuit, it is also possible to provide the cancelling means90 between the air flow rate detecting parts 48, 58 and thecharacteristic adjusting circuits 61, 62.

What is claimed is:
 1. A heating resistor type air flow rate measuringapparatus for measuring an air flow rate with a heating resistorinstalled in an air passage, comprising: a couple of heating resistorsinstalled in the air passage, each of said heating resistors outputtinga respective output signal; wherein a compensated air flow rate signalis output irrespective of the flow direction by correcting a differencevalue between the output signal of one heating resistor and the outputsignal of the other heating resistor; wherein a heater generating heatindependently on an air flow rate is inserted between a couple ofheating resistors used for measuring an air flow rate; and said coupleof heating resistors are placed at positions occupying an upper streamside and a down stream side with respect to an air flow.
 2. A heatingresistor type air flow rate measuring apparatus in which a forward flowdetection signal is detected from a heating current necessary to heat aforward flow heating resistor installed in an air passage to apredetermined temperature, and a backward flow direction signal isdetected from a heating current necessary to heat a backward flowheating resistor installed in the air passage to a predeterminedtemperature, comprising: a canceling means for canceling a differentialmode noise included in each of the detection signals by adding analternating current component of the backward flow detection signal tothe forward flow detection signal and adding an alternating currentcomponent of the forward flow detection signal to the backward flowdetection signal.
 3. A heating resistor type air flow rate measuringapparatus provided with a pair of air flow rate detecting parts fordetecting heating currents necessary to heat a forward and a backwardflow heating resistor installed in an air passage to a predeterminedtemperature, respectively, as a forward flow detection signal and abackward flow detection signal, in order to output an air flow ratesignal including a directional component of the air flow in the air eachpassage by using detection signal, further comprising: a canceling meansfor canceling differential mode noises included in the forward and thebackward detection signals by adding an alternating current component ofthe backward flow detection signal to the forward flow detection signaland adding an alternating current component of the forward flowdetection signal to the backward flow detection signal, and outputtingthe forward and the backward low cancellation signals; wherein an airflow rate signal is output by using the forward and the backward flowcancellation signals instead of the forward and the backward flowdetection signals.
 4. A heating resistor type air flow rate measuringapparatus according to claim 2, wherein said cancelling means includes asignal comparing means for determining a direction of the air flow bythe comparison of large and small of the forward and the backward flowdetection signals, and a signal selecting means for selecting one of theforward and the backward flow cancellation output signals on the basisof the result of determination.
 5. A heating resistor type air flow ratemeasuring apparatus according to claim 4, wherein said cancelling means,said signal comparing means and said signal selecting means isconstructed so as to be included in a semiconductor electronic circuitof a fuel injection amount control unit for controlling the amount offuel injection to an internal combustion engine by using the air flowrate signal.
 6. A heating resistor type air flow rate measuringapparatus comprising: a pair of air flow rate detecting parts fordetecting heating currents necessary to heat a forward and a backwardflow heating resistor installed in an air passage to a predeterminedtemperature, respectively, as a forward detection signal and a backwardflow detection signal, a signal comparing means for determining thedirection of the air flow in the air passage by the comparison of largeand small of the forward and the backward flow detection signals, asignal selecting means for selecting one of the forward and the backwardflow detection signals on the basis of the result of determination, anda differential amplifying circuit for switching and inputting theforward and the backward flow detection signals, adding an alternatingcurrent component of the backward flow detection signal to the inputforward flow detection signal, and switching and outputting either oneof an output signal higher than a reference voltage in proportion to theadded signal and an output signal lower than the reference voltage inproportion to the input backward flow detection signal; wherein an airflow rate signal including a directional component of the air flow isoutput by using the output signal from the differential amplifyingcircuit.
 7. A heating resistor type air flow rate measuring apparatuscomprising: a pair of air flow rate detecting parts for detectingheating currents necessary to heat a forward and a backward flow heatingresistor installed in an air passage to a predetermined temperature,respectively, as a forward flow detection signal and a backward flowdetection signal, a signal comparing means for determining the directionof the air flow in the air passage by the comparison of large and smallof the forward and the backward flow detection signals, a signalselecting means for selecting one of the forward and the backward flowdetection signals on the basis of the result of determination, and adifferential amplifying circuit for switching and inputting the forwardand the backward flow detection signals, inverting the phrase of analternating current component of the forward flow detection signal andadding the resultant signal to the input backward flow detection signal,and switching and outputting either one of an output signal higher thana reference voltage in proportion to the forward flow detection signaland an output signal lower than the reference voltage in proportion tothe added signal; wherein an air flow rate signal including adirectional component of the air flow is output by using the outputsignal from the differential amplifying circuit.
 8. A heating resistortype air flow rate measuring apparatus for measuring an air flow ratewith a heating resistor installed in an air passage, comprising: acouple of heating resistors installed in the air passage, each of saidheating resistors outputting a respective output signal; wherein acompensated air flow rate signal is output irrespective of the flowdirection by correcting a difference value between the output signal ofone heating resistor and the output signal of the other heatingresistor; wherein said correcting of the difference value is performedby multiplying a constant value or a variable constant determined inresponse to an air flow rate and the difference value between outputsignals of said two heating resistors; wherein a heater generating heatindependently on an air flow rate is inserted between a couple ofheating resistors used for measuring an air flow rate; and said coupleof heating resistors are placed at positions occupying an upper streamside and a down stream side with respect to an air flow.
 9. A heatingresistor type air flow rate measuring apparatus for measuring an airflow rate with a heating resistor installed in an air passage,comprising: a couple of heating resistors installed in the air passage,each of said heating resistors outputting a respective output signal;wherein a compensated air flow rate signal is output irrespective of theflow direction by correcting a difference value between the outputsignal of one heating resistor and the output signal of the otherheating resistor; wherein said two heating resistors are placed atclosed positions where said two heating resistors interfere thermallywith respect to air flow; wherein a heater generating heat independentlyon an air flow rate is inserted between a couple of heating resistorsused for measuring an air flow rate; and said couple of heatingresistors are placed at positions occupying an upper stream side and adown stream side with respect to an air flow.
 10. A heating resistortype air flow rate measuring apparatus for measuring an air flow ratewith a heating resistor installed in an air passage, comprising: acouple of heating resistors installed in the air passage, each of saidheating resistors outputting a respective output signal; wherein acompensated air flow rate signal is output irrespective of the flowdirection by correcting a difference value between the output signal ofone heating resistor and the output signal of the other heatingresistor; wherein an output signal of a heating resistor placed at aupper stream of an air flow is used as said reference output signal ofsaid heating resistor used as a reference; wherein a heater generatingheat independently on an air flow rate is inserted between a couple ofheating resistors used for measuring an air flow rate; and said coupleof heating resistors are placed at positions occupying an upper streamside and a down stream side with respect to an air flow.
 11. A heatingresistor type air flow rate measuring apparatus for measuring an airflow rate with a heating resistor installed in an air passage,comprising: a couple of heating resistors installed in the air passage,each of said heating resistors outputting a respective output signal;wherein a compensated air flow rate signal is output irrespective of theflow direction by correcting a difference value between the outputsignal of one heating resistor and the output signal of the otherheating resistor; wherein said two heating resistors include twoindependent drive circuits; wherein a heater generating heatindependently on an air flow rate is inserted between a couple ofheating resistors used for measuring an air flow rate; and said coupleof heating resistors are placed at positions occupying an upper streamside and a down stream side with respect to an air flow.
 12. A heatingresistor type air flow rate measuring apparatus for measuring an airflow rate with a heating resistor installed in an air passage,comprising: a couple of heating resistors installed in the air passage,each of said heating resistors outputting a respective output signal;wherein a compensated air flow rate signal is output irrespective of theflow direction by correcting a difference value between the outputsignal of one heating resistor and the output signal of the otherheating resistor; wherein output signals obtained by said two heatingresistors are adjusted by a circuit so as to be identical to each otherwith respect to an air flow from a certain direction; wherein a heatergenerating heat independently on an air flow rate is inserted between acouple of heating resistors used for measuring an air flow rate; andsaid couple of heating resistors are placed at positions occupying anupper stream side and a down stream side with respect to an air flow.13. A heating resistor type air flow rate measuring apparatus having acouple of heating resistors thermally interfering with each other withrespect to an air flow, each heating resistor including an independentdrive circuit, wherein respective output signals of the two heatingresistors, which correspond to an air flow rate, being adjusted so thatboth output signals are the same with respect to an air flow in onedirection and said output signals of respective heating resistors beingcompensated irrespective of the flow direction by correcting adifference value of said output signals of said two heating resistors;wherein a filter is mounted between an output part for an output signaland an input part for an air flow rate signal in a control unit for afuel injection control.
 14. A heating resistor type air flow ratemeasuring apparatus having a couple of heating resistors thermallyinterfering with each other with respect to an air flow, each heatingresistor including an independent drive circuit, wherein respectiveoutput signals of the two heating resistors, which correspond to an airflow rate, being adjusted so that both output signals are the same withrespect to an air flow in one direction and said output signals ofrespective heating resistors being compensated irrespective of the flowdirection by correcting a difference value of said output signals ofsaid two heating resistors; wherein a fuel injection control isperformed in response to an output signal.
 15. A heating resistor typeair flow rate measuring apparatus provided with a pair of air flow ratedetecting parts for detecting heating currents necessary to heat aforward and a backward flow heating resistor installed in an air passageto a predetermined temperature, respectively, as a forward flowdetection signal and a backward flow detection signal, in order tooutput an air flow rate signal including a directional component of theair flow in the air each passage by using detection signal, furthercomprising: a canceling means for canceling differential mode noisesincluded in the forward and the backward detection signals by adding analternating current component of the backward flow detection signal tothe forward flow detection signal and adding an alternating currentcomponent of the forward flow detection signal to the backward flowdetection signal, and outputting the forward and the backward lowcancellation signals; wherein an air flow rate signal is output by usingthe forward and the backward flow cancellation signals instead of theforward and the backward flow detection signals; wherein said cancellingmeans includes a signal comparing means for determining a direction ofthe air flow by the comparison of large and small of the forward and thebackward flow detection signals, and a signal selecting means forselecting one of the forward and the backward flow cancellation outputsignals on the basis of the result of determination.
 16. A heatingresistor type air flow rate measuring apparatus according to claim 15,wherein said cancelling means, said signal comparing means and saidsignal selecting means is constructed so as to be included in asemiconductor electronic circuit of a fuel injection amount control unitfor controlling the amount of fuel injection to an internal combustionengine by using the air flow rate signal.